1. Field of the Invention
The invention relates to energy curable coating resins made of poly(ethylene)terephthalate resin useful as binder materials in coating compositions and to a process for producing these compositions.
2. Background of the Invention
Material recycling and resource recovery are common practices in all but a few industries. Responding with passion and prudence to the new awareness of the fragility of our ecosystem, society has set about the rectification of past material excesses by falling upon the more ubiquitous materials used in modern civilization for corrective recovery. There is none more ubiquitous than plastics and no more ubiquitous plastics than polyolefins and polyesters such as polyethylene terephthalate (PET).
PET is a high molecular weight linear polymer known in the fiber, film and plastic industries for more than forty years. The polymer is prepared from dimethyl terephthalate and ethylene glycol via a transesterification reaction, which is typically carried out with a basic catalyst such as antimony trioxide. The resulting material is a high melting solid exhibiting superior clarity, excellent mechanical properties, good thermal stability and resistance to common solvents. It can be readily melt spun to fiber, cast as film, extruded, blow or injection molded into a wide variety of high value articles. Compared to polyolefins, PET is a relatively expensive resin as the monomers are derived from para xylene and ethylene. Nonetheless, the properties of PET are so superior that the resin successfully challenges polyolefins as the material of choice for the manufacture of clothing, carpeting, industrial products, soft drink bottles, cast and injection molded articles.
PET's utility and success places discarded PET materials high on the list of those discarded plastics and fibers that must be collected and recycled. But it is rare that reclaimed or recycled PET is used to replace virgin resin in processing plastic materials. This is because the process seldom can be carried out without some degree of degradation of the reclaimed polymer. This limits the amount of reclaimed PET that can be used to mold resins or film and all but eliminates the use of reclaimed PET in fibers. Consequently, the plastic industry has been hard at work to find better ways to employ reclaimed PET to take advantage of its superior mechanical and chemical properties.
Of course, resins of many varieties such as solvent-borne, ultra-violet (UV) and electron beam (EB) energy curable resins, are used in enormous quantities for a multitude of applications in the organic coatings industries, i.e. as protective, barrier and insulating coatings, adhesives, sealants, dye and ink coatings, etc. However, virgin PET has garnered little of the coating market for resins because of its cost. The resins commonly used are vinyl polymers, particularly acrylates, polyvinyl chloride and polyvinyl acetate, epoxies, polyolefins, polyurethanes, aliphatic polyesters, phenolics, and the like. They are used in coating formulations as pre-formed high molecular weight polymers such as acrylate latex or, more typically, as low molecular weight oligomers which are crosslinked to form a matrix subsequent to being applied on a substrate. Despite its otherwise superior mechanical and chemical properties, virgin PET cannot compete with the foregoing resins of choice in price or availability. In the organic coatings industry it is desirable to have readily available UV/EB crosslinkable PET oligomers.
At least with respect to price, reclaimed PET can compete with some of the more commonly used resins in coatings. For instance, the current price of reclaimed PET is far below the price of acrylates, particularly polymethylmethacrylate, which are commonly incorporated as the resin of choice for UV/EB curable coatings where high performance dictates the choice of resin. But for the absence of a readily UV/EB crosslinkable reclaimed PET oligomer, organic coatings would appear to be a viable market use for reclaimed or recycled PET.
S. Lee et al., Polyurethanes Synthesized from Polyester Polyols Derived from PET Waste, Journal of Applied Polymer Science, Vol. 55, pgs. 1271-73 (1995), teaches the depolymerization of PET waste by glycolysis to obtain oligomeric diols.
U. R. Vaidya et al., Unsaturated Polyesters from PET Waste, Journal of Applied Polymer Science, Vol. 34, pgs 235-46 (1987), teaches the glycolysis of PET waste with propylene glycol at different weight ratios.
U.S. Pat. No. 5,552,478 teaches preparing oligomeric glycols from recycled PET by digesting a recyled PET feedstock with asymmetrical glycols then reacting the resulting mixture with acids having fewer than 12 carbon atoms.
It is an object of the present invention to provide a low cost PET oligomer for use in coating formulations and as a binder in UV and EB curable coatings.
Another objective of the invention is to produce the resin primarily from recycled or reclaimed PET.
Other objects and advantages of the invention will become apparent from the following description of embodiments.